1. Field of the Invention
This invention relates to a heater control apparatus for oxygen concentration sensor heating for detecting an oxygen concentration contained in exhaust gases of an internal combustion engine.
2. Description of the Related Art
A technique for making purification of the exhaust gases and improvements in fuel consumption by detecting the oxygen concentration contained in the exhaust gases of the internal combustion engine and performing feedback control on an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine in response to the detected oxygen concentration is well known, and the technique is widely used in the internal combustion engine for vehicle. An exhaust gas sensor for detecting the oxygen concentration needs to stabilize detection characteristics of the oxygen concentration by maintaining a temperature at an activation region and for this purpose, a heater control apparatus for passing a current through a heater built into the sensor and controlling a passage current to hold a temperature of the sensor at a predetermined value is used and various techniques are disclosed as this control method.
For example, a technique for detecting a resistance value of a heater from voltage and current values of the heater and calculating a temperature of the heater from this resistance value to see an activation state of an exhaust gas sensor and also performing duty control of a passage current for temperature control and speeding up activation by continuous passage of current at the time of low temperature (for example, at the time of starting of an internal combustion engine) is disclosed in JP-A-8-313476. Also, a technique for detecting a resistance value of an exhaust gas sensor itself with high accuracy to detect the temperature and also switching a voltage applied to the exhaust gas sensor at the time of detecting an oxygen concentration to a voltage for resistance detection at a predetermined time constant in order to reduce this detection time and detecting an internal resistance value of the exhaust gas sensor from a change state of voltage and current at that time is disclosed in JP-A-9-292364.
Further, a technique in which when temperature control of an exhaust gas sensor is performed by turning on or off a heater, stabilization of an element temperature of the exhaust gas sensor and accuracy of air-fuel ratio control are improved while increasing a life of the heater by providing delay time in on-off control and operating the delay time using a running state of an internal combustion engine (for example, rotational speed) as parameters to reduce the number of on-off actions of the heater is disclosed in JP-A-7-119736. Furthermore, a technique for holding accuracy of a heater temperature and improving durability and detection accuracy of an oxygen concentration by setting a desired value of a heater resistance in response to power consumption of a heater and an exhaust gas flow and controlling an applied voltage so that a resistance value of the heater becomes equal to this desired value is disclosed in JP-A-1-172746.
As indicated in each the conventional example, the variety of techniques for speeding up the activation at the time of starting while controlling the temperature to improve the durability and the detection accuracy of the oxygen concentration by performing current-carrying control to the heater are disclosed, but a power source for heating the heater of the exhaust gas sensor used in a vehicle is a battery mounted in the vehicle and a change in voltage of this battery is large during operation of the vehicle, so that it takes time to perform initial heating at the time of a low voltage and if case of improving a heating capability in order to cope with this, burning of the heater or driving elements was caused at the time of a high voltage or troubles could not be avoided in temperature control by the conventional current-carrying control. Particularly, in case that a state of connection between the battery and a charging generator for charging this battery became worse, a voltage of the charging generator might increase transiently and a relatively high voltage from this charging generator might be applied to the heater and in such a case, the burning troubles of the heater or the driving elements could not be avoided.
The invention is implemented to solve such problems, and it is an object of the invention to obtain a compact and cheap heater control apparatus for exhaust gas sensor in which troubles such as burning do not occur at the time of overvoltage even in case of using a heater with high heating capability and a sufficient preventive safety measures are taken.
A heater control apparatus for exhaust gas sensor according to the invention comprises an exhaust gas sensor which is provided in an exhaust passageway of an internal combustion engine and detects an oxygen concentration in exhaust gases, a heater for heating this exhaust gas sensor to a predetermined temperature, a power source including a battery for supplying heating electric power to this heater and a charging generator for charging this battery, a switching element inserted into a circuit for supplying electric power to the heater from this power source, control means including a microcomputer for performing current-carrying control of the switching element so as to maintain a temperature of the exhaust gas sensor at a predetermined value, and an overvoltage detection element for outputting an abnormal detection signal when a voltage of the power source exceeds a predetermined value, and it is constructed so that the switching element is interrupted to stop the passage of current to the heater by the abnormal detection signal of the overvoltage detection element when a voltage of the charging generator abnormally increases due to disconnection etc. of the battery of the power source.
Also, the heater control apparatus comprises an exhaust gas sensor which is provided in an exhaust passageway of an internal combustion engine and detects an oxygen concentration in exhaust gases, a heater for heating this exhaust gas sensor to a predetermined temperature, a power source including a battery for supplying heating electric power to this heater and a charging generator for charging this battery, a switching element inserted into a circuit for supplying electric power to the heater from this power source, control means including a microcomputer for performing current-carrying control of the switching element so as to maintain a temperature of the exhaust gas sensor at a predetermined value, circuit abnormal detection means for detecting circuit abnormality such as short-circuit troubles of a current carrying circuit to the heater and outputting an abnormal detection signal, an overvoltage detection element for outputting an abnormal detection signal when a voltage of the charging generator increases and exceeds a predetermined value due to disconnection etc. of the battery of the power source, and a second switching element which is inserted into a circuit for supplying electric power to the heater from this power source and responds to the abnormal detection signal of the circuit abnormal detection means and also responds to the abnormal detection signal of the overvoltage detection element and interrupts a current flowing through the heater.
Further, the control means performs current-carrying control of the switching element at a switching ratio in response to a value of a source voltage.
Furthermore, a gate element for driving the switching element for interrupting a current flowing through the heater by the abnormal detection signal is provided and the abnormal detection signal is applied to the gate element directly without passing the control means and the switching element is constructed so as to interrupt the current by operations of the gate element.
Moreover, the heater control apparatus further comprises storage means for encoding and storing signal contents of the abnormal detection signal when the overvoltage detection element or the circuit abnormal detection means outputs the abnormal detection signal, and display means for displaying the fact that the abnormal detection signal has been outputted.
In addition, the control means measures an internal resistance value before start of a run and an internal resistance value during the run of the exhaust gas sensor and calculates a temperature of the exhaust gas sensor from a ratio between both of the resistance values.
Also, the control means reads a voltage value across both ends of the switching element at opening of the switching element and a current value flowing through the switching element at closing in a time division manner and calculates a resistance value of the heater from the voltage value and the current value and calculates a temperature of the exhaust gas sensor as a function of this resistance value of the heater.